This invention relates to diffusers of centrifugal fluid machines, and more particularly it is concerned with a diffuser of a centrifugal fluid machine, specifically centrifugal compressor, capable of achieving a wide operation range and increased operation efficiency.
A vaneless diffuser used with a centrifugal compressor of the prior art comprises a pair of diffuser plates (disks) defining therebetween a fluid channel. A flow of fluid of high velocity issuing from the impeller has its energy of velocity converted to energy of pressure at the diffuser, to be collected and discharged through the outlet port after continuously recovering pressure in the scroll casing disposed downstream of the diffuser plates. Particularly in an impeller of high specific velocity which handles a fluid flowing in high flow rate, the flow angle .alpha..sub.2 and the radial component C.sub.2m of the absolute velocity of the fluid at the outlet of the impeller have generally tended to show a markedly non-uniform (distorted) distribution from the shroud toward the hub as shown in FIGS. 1 and 2, because of the relatively large width of the outlet of the impeller. A distortion in the fluid flow at the inlet of the diffuser causes a marked reduction in the performance of the difuser as shown in FIG. 3 which shows the results of tests conducted on a two-dimensional diffuser in which Cp represents the ratio of the recovery of static pressure to the dynamic pressure at the diffuser inlet, and Bf represents the quantity (1--the ratio of fluid channel effective cross-sectional area to the geometrical cross-sectional area of fluid channel). The subscript ref indicates a reference value. The same results as shown in FIG. 3 could be obtained in tests on a radial type diffuser used in a centrifugal fluid machine.
As set forth hereinabove, the diffuser of a centrifugal compressor of the prior art has had nonuniformity in the flow of a fluid immediately after the fluid clears the impeller, thereby causing a marked reduction in performance. To obviate this disadvantage, we have made a proposal to provide, on at least one of the diffuser plates at the inlet portion of the diffuser, a plurality of flow guide vanes arranged in the form of a circular cascade and each having a height that does not exceed one-half the width of the fluid channel, as disclosed in U.S. Ser. No. 171,268, filed July 22, 1980.
This proposal has proved to have effect in rendering uniform the flow of the fluid clearing the impeller at the inlet of the diffuser. However, the proposed flow guide vanes have been found to have no effect in avoiding the occurrence of secondary flow in the lower portion of the vaneless diffuser. The reason is that in a flow of a viscous fluid, even if the fluid is caused to flow in a uniform pattern in the inlet portion of the diffuser, secondary flow is induced to occur in the outlet portion of the diffuser disposed on the downstream side because of the presence of the boundary layer which develops therein after being formed in the inlet portion.